1. The Field of the Invention
The present invention relates to an optical glass, to the use of such a glass, to optical elements or preforms of such optical elements, to a method for producing such optical elements and to optical parts or optical components made of such optical elements.
2. The Related Art
In recent years, the market trend for both optical and optoelectronic technologies in the application fields of imaging, projection, telecommunication, optical communications engineering and laser technology has increasingly been tending towards miniaturization. This can be seen from the end products, and it also requires ever smaller dimensions for the individual parts and components.
For the producers of optical glasses, this firstly means a significant reduction of the material volumes ordered, despite increased production numbers. Furthermore, there is an increasing cost pressure on the pail of the manufacturers for whom much more waste is incurred as a percentage in relation to the product when producing much smaller component sizes from block or ingot glass, and the processing of such small pieces furthermore often entails greatly increased outlay.
For these reasons, there are increasing demands from manufacturers for blank-pressed small components and therefore, as their precursors, also for preforms close to final geometry for re-pressing, so-called “precision gobs”. These precision gobs are fully fire-polished, semifree- or free-formed glass portions which can be produced in various ways.
One production method for gobs is the pearl spraying method. No controlled positioning is required in this method; instead, the required size fraction is separated for example by screening. The residual fraction does not need to be discarded, but can be reused as highly pure cullet which can be re-melted particularly well. Furthermore, with this method which is particularly easy to carry out in terms of technology and staff, large batch numbers are achieved within a short time.
In contrast to this, direct pressing close to final geometry, on which greater value is to be placed in the supply chain, entails the problem of economic viability. Although this method can flexibly compensate for the small glass melt volumes, distributed over a large batch number of small material pieces, by short setting up times, with small geometries the value creation cannot however come only from the material value because of the lower cycle time/batch number in comparison with gob spraying. The products must leave the press in a state which is ready for system incorporation (“ready-to-clamp”) without requiring elaborate adjustment, cooling and/or cold reprocessing. For this reason, owing to the high geometrical accuracies required, it is necessary to use precision equipment with high-quality and therefore expensive mold materials. The service lives of the molds make a huge impact on the economic viability of the products or materials. One extremely important factor when considering the service lives is the operating temperature, which is dictated here by the viscosity of the materials to be pressed. For glass, this means that the lower the transition temperature Tg is, the longer the mold service lives are when pressing this glass and therefore the greater the profit margin is. This explains the glass producers' demand for so-called “low-Tg glasses”, i.e. glasses that can be processed at temperatures which are as low as possible.
In addition, melt process technology has recently reported more demand for “short” glasses, i.e. for glasses whose viscosity varies greatly with temperature. For processing, this behavior has the advantage that the hot forming times, i.e. the mold closure times, can be reduced. In this way, on the one hand the throughput is increased (cycle time reduction), and on the other hand it conserves the mold material, which has an extremely positive impact on the overall production costs. Owing to the more rapid cooling which this allows, it also makes it possible to process glasses with a greater susceptibility to crystallization than in the case of correspondingly longer glasses, and it avoids prenucleation which could be problematic in subsequent secondary hot forming steps. As a result, this in turn means that these materials are also very suitable for rod, tube and fiber production, besides gob production and direct pressing from the melt.
Besides these properties, which are important for hot forming, such glasses must furthermore have good properties for cold reprocessing so that they can be sold viably on the world market, since as before some of the material is processed in the conventional way by cutting, grinding and polishing, particularly for parts and components with sizeable geometries or dimensions. To this end, the glasses must have a sufficiently good chemical stability or chemical resistance. If this were not the case, grinding or polishing agents and contact with the predominantly aqueous media of the cleaning baths would damage the very precisely processed surfaces. Scratches, efflorescences and discolorations would occur. Moderate thermal expansion values are likewise required, which ensure that stress cracks or strained materials are not produced in processing steps with intense thermal shocks. The hardness (here: Knoop hardness) should also not be too great, in order to keep machine processing times within viable limits.
The prior art relevant to the invention is summarized in the following documents:
DE 10 2005 005 994SchottDE 10 239572SchottJP 2 124 743ASumitaUS 2004 0 138 043HoyaDE 1 089 934SchottJP 60 171 244AOharaJP 63 011 544AHoyaUS 5 022 921CorningJP 2007 070 194OharaJP 9 278 479AOhara
According to these, although glasses can be produced with a similar optical position and/or roughly comparable chemical composition, these glasses nevertheless exhibit considerable disadvantages in comparison with the glasses according to the invention.
DE 10 2005 005 994 describes glasses with a comparable optical position. These, however, are glasses of the aluminoborosilicate glass system with a different physicochemical property profile. Owing to the high proportion of conventional glass-forming substances (sum of SiO2, B2O3, Al2O3 50-71 wt. %) and the absence of phosphate, in spite of their very high alkaline earth metal oxide content the glasses exhibit glass transition temperatures of about 500° C., the lowest Tg referred to by way of example being 470° C.
DE 10 239 572 describes lithium oxide- and germanate-free zinc phosphate glasses with a comparable optical position. The Tgs lie in the same range of around 400° C., which is suitable for hot forming processes close to final geometry (for example blank pressing). Owing to the absence of GeO2 which stabilizes the matrix, however, these glasses have only a very low acid resistance.
The glasses disclosed in JP 2 124 743A have the same disadvantage. Owing to the lack of a germanate component, they had an inferior acid resistance class compared with the glasses according to the invention, and are therefore less suitable for further mechanical processing.
The glasses disclosed in US 2004/0138043 also have corresponding disadvantages. Without germanate the acid resistance class is too low, and the use of all alkali metal oxides in parallel, probably aiming to achieve the mixed alkali effect known from silicate systems, leads to an increased turbidity risk in comparison with the glasses according to the invention. Owing to the Bi2O3 content of at least 0.5 mol %, the described glasses furthermore lose transmission at the blue edge and contain a highly redox-sensitive component which leads to great outlay in the production process.
The glasses disclosed in JP 9 278 479A and U.S. Pat. No. 5,022,921 also contain no germanate. Both documents use conventional glass-forming substances, for example Al2O3, or hardness-increasing components, for example La2O3, to improve the chemical resistance, although these either (in the case of glass-forming substances) provide no improvement corresponding to the resistance stabilization of the glasses according to the invention or (in the case of hardness-increasing components) reduce the crystallization stability of the glasses owing to network modification without stabilization by GeO2.
The same also applies to the glasses described in JP 2007-070 194 and JP 63-011 544, all the more so since these are not zinc-phosphate glasses but alkaline earth metal aluminophosphate glasses, the viscosity-temperature profile of which is less suited to the requirements of precision hot forming, i.e. the glasses are longer.
DE 1 089 934 and JP 60-171 244 A describe germanate-free glasses of the borophosphate glass system with an intrinsically much higher Tg. Although these have a good chemical resistance, owing to their not correspondingly optimized viscosity-temperature profile they are unsuitable for processing in precision hot forming methods.